The velocity of water current in a river changes with distance along the perpendicular to the river according to the law u = 2u0 dy for 0 ≤ y ≤ d2 where u0 = velocity in the mid-stream d = width of the river = 2u0d (d − y) for d2 ≤ y ≤ d. A boat travels from a point O on one block of the river to the opposite bank and its steering angle is constantly changed to keep its relative velocity perpendicular to the river current. Calculate the time in which the boat will reach the other bank. The velocity of the boat in still water is u0.